Recently, a major cause of death in advanced countries has shifted from infectious diseases to adult diseases. In the midst of such change in the disease structure cancer is a particularly important disease that commonly ranks high among the major causes of death in many countries. In Japan, for example, the annual number of people who die from cancer is over 300,000. This is nearly twice as high as the number of people who die from heart diseases. Therefore, it is an important research challenge to provide cancer therapeutic technology.
Generally, surgical treatment, physicochemical treatment, drug treatment and the like are used for cancer therapy. Physicochemical treatment includes radiation therapy, particle beam (heavy charged particle beam) therapy, and thermotherapy. Many chemotherapeutic agents used for drug treatment have been put into practical use. Further, therapeutic effects of new approaches such as vaccine therapy and cellular immunotherapy have been confirmed. However, it is still an important research challenge to provide methods for treating cancer. In particular, techniques to prevent malignant transformation of cancer or methods to treat malignant cancer will contribute greatly to the medical treatment of cancer if they can be provided.
Generally, the mechanisms of malignant cancer transformation can be explained by growth, infiltration, and metastasis of cancer cells. In other words, when grown cancer cells metastasize by infiltration, the cancer is regarded as malignant. Cancer growth usually involves angiogenesis. Therefore, angiogenesis is also an important mechanism underlying malignant cancer transformation.
A series of the underlying mechanisms of malignant cancer transformation are explained in general as follows. First, cancer cells are grown and released from a primary site. The released cancer cells travel through lymphatic flow and blood flow and infiltrate other tissues. Metastasis is established when the infiltrated cancer cells start growing again. The growth of cancer cells requires angiogenesis. Angiogenesis also has great significance in the release of cancer cells from a primary site into the blood flow. It is considered that if any of these mechanisms is inhibited, the malignant transformation of cancer can be prevented. Thus, cell growth, infiltration, metastasis, and angiogenesis are important therapeutic targets in the treatment of cancer, especially of malignant cancer.
More than 90% of cancer occurs in epithelial cells. Epithelial cells maintain a strong sheet structure by adhering to one another through E-cadherin. Therefore, migration of epithelial cells is restricted. In malignant cancer, the easy release from a primary site is attributed to the weak intercellular adhesiveness of cancer.
However, malignant transformation cannot be explained only by migration property of cells. Generally, solid cancer tissues are wrapped in an extracellular matrix (ECM). In order for cancer to metastasize, cancer cells should pass through the ECM by certain mechanisms. ECM is mainly composed of the following components: collagen, fibronectin, laminin, proteoglycan, and elastin.
The percentage of each component and other components differs from tissue to tissue. Furthermore, each of these components has some subtypes. However, the most essential component of ECM that is common to multiple tissues is collagen. It has been revealed that malignant cancer migrates within tissues by producing an enzyme that degrades the collagen in ECM. Generally, collagen is a stable molecule and often possesses resistance to various proteases. Therefore, destruction of ECM by protease is an important condition for cancer to be malignant. A number of proteases involved in the decomposition of ECM have been identified. Proteases that participated in the decomposition of ECM are referred to as matrix metalloproteinases (MMPs). MMPs are seen as an important target molecule for inhibiting cancer metastasis. In the living body, protease inhibitors such as tissue inhibitor of metalloproteinase (TIMP) and α-2-macroglobulin (α2M) control the activity of MMPs.
Meanwhile, some MMPs change from a latent form to an active form upon digestion by another protease. For example, MMP-1 and MMP-3 are known to become an active form with plasmin. Plasmin acquires protease activity through the activation of plasminogen which requires the plasminogen activator (PA). There are two types of PA: urokinase type (uPA) and tissue type (tPA). tPA is a molecule that acts mainly on fibrinolytic system and is used as a thrombolytic agent. On the other hand, it has been pointed out that uPA may be involved in the infiltration and metastasis of cancer.
As discussed above, angiogenesis is an important mechanism underlying the malignant transformation of cancer. Therefore, like MMPs, angiogenesis in cancer tissues is also considered an important target in cancer therapeutic strategy. In fact, it is known that the expression of vascular endothelial growth factor (VEGF) is elevated in many cancers. Furthermore, it has been pointed out that the ECM-decomposing proteases have an important role not only in the cell migration as described above but also in angiogenesis. For example, proteases decompose ECM to make room for angiogenesis. As described above, various protease activity control systems are intricately involved in the malignant transformation of cancer.
In the living body, protease activities are generally controlled by the binding of proteases to their inhibitors. Furthermore, the function of some protease activating factors such as PA is also controlled by their inhibitors. For example, it has been shown that uPA as described above forms a complex with protein C inhibitor (PCI) in urine (Non-Patent Document 1). It is also known that PCI inhibits the activity of uPA (Non-Patent Document 2).
PCI is a protease inhibitor identified as an inhibitory factor of protein C, which is an anticoagulant protease (Non-Patent Documents 3 and 4). It belongs to the serine protease inhibitor (SERPIN) family, and has been found to have inhibitory activities against thrombin, factor Xa, factor XIa, plasma kallikrein, uPA, and the like. Plasminogen activator inhibitor 3, which was identified as an inhibitor of PA, is the same molecule as PCI.    [Non-Patent Document 1] Stump D. C. et al., J. Biol. Chem. 261: 12759-66, 1986    [Non-Patent Document 2] Stief T. W. et al., Biol. Chem. Hoppe Seyler 368: 1427-33, 1987    [Non-Patent Document 3] Marlar R. A. et al., J. Clin. Invest. 66: 1186-9, 1980    [Non-Patent Document 4] Suzuki K. et al., J. Biol. Chem. 258: 163-8, 1983